Heretofore, fuels or propellants in gaseous, liquid and solid forms have been used for space propulsion units. In a general way, a gaseous propellant is highly pressurized and stored in a highly dense state, because the gaseous propellant under natural conditions requires a relatively large container volume. Thus, a storage container (tank) and associated components, such as pipes and valves, are essentially designed to have sufficient pressure resistance and structural strength to withstand such a high pressure. This causes a problem about increase in weight. Moreover, the high pressure is highly likely to cause failures, such as gas leakage from the valve or locking of the valve. A liquid propellant needs to use a high-pressure transfer system even though it originally has a high density, and therefore involves the same problem as that in the gaseous propellant. Further, a thruster using a high-pressure system has a problem about the need for performing a propellant-charging operation as a hazardous job before launch. A solid propellant originally has a high density, and exhibits excellent storage performance without the need for a high-pressure system. On the other hand, the solid propellant has a problem that, once ignited, a propulsive action cannot be stopped until being completely consumed, and a thrust cannot be on/off-controlled (interruption and restart of a thrusting operation) or adjusted. An explosive serving as the solid fuel is a flammable material subject to a fire ban in handling, and therefore has poor handleability on the ground. With a view to improving such disadvantages of the solid fuel, there have been made researches on a technique for storing a solid fuel in the form of a plurality of pieces divided on the basis of a volume required for each combustion, and igniting each of the pieces according to need (see, for example, the following Non-Patent Publication 1). However, this technique has a disadvantage that the sold fuel occupies a relatively large area depending on a required number of combustions.
A small-size thruster having difficulty in obtaining a high specific thrust (or specific impulse) needs a larger volume of propellant to generate a required ΔV. A weight of a section for storing a propellant is apt to increase in proportion to a volume of the propellant. Thus, it is important for a small-size thruster to reduce a dry weight of thruster components other than a fuel. In view of the above technical background, there has been proposed a device designed to emit a laser beam onto a solid propellant applied on a surface of a film so as to generate an ablation jet (see, for example, the following Patent Publication 1). A technique of emitting a laser beam from a back surface of the film as disclosed in the Patent Publication 1 can prevent a body and optics system of a laser device from being contaminated by jet substances, and has a certain level of effectiveness in this point. On the other hand, this device has a disadvantage of causing an increase in dry weight of a propulsion unit, because a weight of the film will increase in proportion to a volume of the propellant, and the increased weight of the film will be included in a weight of the propellant storage section despite of no contribution to thrust.
In the device disclosed in the Patent Publication 1, no nozzle is used for ablation jets, and therefore it is difficult to effectively generate a thrust. Moreover, a vaporized propellant is likely to spread and re-solidify, resulting in causing contamination of surroundings. In a space satellite designed to accurately adjust infrared characteristics on a surface thereof so as to control a temperature of the surface, a surface contamination causes serious evils. Thus, the above phenomenon is a critical problem.
As a solid propellant-based propulsion device utilizing no chemical reaction, there has been known one type, so-called “pulsed plasma thruster (PPT)” (see, for example, the following Patent Publication 2). While various materials have been tried as a solid propellant, PTFE (Polytetrafluoroethylene (Teflon®)) is commonly used (see, for example, the following Non-Patent Publication 2). This thruster has a disadvantage that a specific thrust cannot be desirably improved due to sublimated gas to be generated with a delay after completion of a pulsed discharge. Thus, a propellant is limited to a specific type having a low level of delayed gas generation. As other technological developments, efforts have been made for a technique of using a liquid propellant (see, for example, the following Non-Patent Publication 3), and a technique of controlling a sublimation quantity based on laser ablation (see, for example, the following Non-Patent Publication 4).
[Patent Publication 1] U.S. Pat. No. 6,530,212
[Patent Publication 2] U.S. Patent Application Publication No. 2003/0,033,797
[Non-Patent Publication 1] S. Tanaka, R. Hosokawa, S. Tokudome, K. Hori, H. Saito, M. Watanabe and M. Esaka, “MEMS-based Solid Propellant Rocket Array Thruster”, ISTS 2002-a-02, Proceedings of the 23 International Symposium on Space Technology and Science, Matsue, 2002, pp. 6-11.
[Non-Patent Publication 2] H. Kamhawi, E. Pencil and T. Haag, “High Thrust-to-Power Rectangular Pulsed Plasma Thruster”, AIAA 2002-3975, Join Propulsion Conference, Indianapolis, Jul. 2002.
[Non-Patent Publication 3] A. Kakami, H. Koizumi and K. Komusasaki, “Performance Study on Liquid Propellant Pulsed Plasma Thruster”, AIAA 2003-5021, Joint Propulsion Conference, Huntsville, Jul. 2003.
[Non-Patent Publication 4] M. Kawakami, W. Lin, A. Igari, H. Horisawa and I. Kimura, “Plasma Behaviors in a Laser-Assisted Plasma Thruster”, AIAA 2003-5028, Joint Propulsion Conference, Huntsville, Jul. 2003.